Basically, it's because the fuel in the (nearly-full) tanks caught fire, which weakened the columns, which bent slightly, which meant that the roof - and other floors- fell in.

We know that from the real-life example, of course. But it's interesting to see it demonstrated here. Not, of course, that this will satisfy the wingnuts who think it was a conspiracy. Their loss.

But sometimes this is what engineering is about: figuring out why things happen after the event. (Other videos on the National Geographic site ask whether controlled demolition could have done the same job, and whether the hijacked planes were replaced by planes carrying explosives. Enjoy.)